Method and system for producing a reservoir through a boundary layer

ABSTRACT

A method for producing a reservoir includes drilling a well into a boundary layer that is hydraulically connected to the reservoir. Reservoir fluid migration from the reservoir to the boundary layer is accelerated and the reservoir fluid is produced through the well.

TECHNICAL FIELD

This disclosure relates generally to the field of recovery of subterranean resources, and more particularly to a method and system for producing a reservoir through a boundary layer.

BACKGROUND

Reservoirs are subterranean formations of rock containing oil, gas, and/or water. Unconventional reservoirs include coal and shale formations containing gas and, in some cases, water. A coal bed, for example, may contain natural gas and water.

Coal bed methane (CBM) is often produced using vertical wells drilled from the surface into a coal bed. Vertical wells drain a very small radius of methane gas in low permeability formations. As a result, after gas in the vicinity of the vertical well has been produced, further production from the coal seam through the vertical well is limited.

To enhance production through vertical wells, the wells have been fractured using conventional and/or other stimulation techniques. Horizontal patterns have also been formed in coal seams to increase and/or accelerate gas production.

SUMMARY

Method and system for producing fluid from a reservoir through a boundary layer are provided. For example, methane or other gas may be produced from a coal reservoir through a permeable boundary layer.

In accordance with one embodiment, a method for producing a reservoir includes drilling a well into a boundary layer that is hydraulically connected to the reservoir. Reservoir fluid migration to the boundary layer is accelerated and the reservoir fluid is produced and/or disposed of through the well.

In one or more specific embodiments, the well may be a cavity well having a cavity formed in the boundary layer. In this and other embodiments, the well may terminate in the boundary layer. Water from the boundary layer and reservoir may be collected in the well and pumped to the surface through a pump having inlet at the level of the boundary layer. The reservoir may be an unconventional or other suitable reservoir. The boundary layer may be a fractured or other suitable layer having a permeability greater than that of the reservoir.

Technical advantages of the system and method may in one or more embodiments include providing accelerated gas production from sub-surface coal, shale and other suitable reservoirs. In a particular embodiment, for example, the boundary layer has a large contact area with the reservoir and a higher permeability than the reservoir. The boundary layer is drawn down to initiate and/or accelerate migration of fluids from the reservoir to the boundary layer through the large contact area. In the higher permeability boundary layer, the gas flows more easily and/or readily to the wells for production to the surface.

The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating one embodiment of a plurality of wells for producing a reservoir through a boundary layer;

FIG. 2 is a cross-sectional view illustrating another embodiment of a plurality of wells for producing a reservoir through overlying and underlying boundary layers;

FIG. 3 is a perspective view illustrating one embodiment of a pattern of wells extending into a boundary layer having a large content area with a reservoir; and

FIG. 4 is a flow diagram illustrating one embodiment of a method for producing a reservoir through a boundary layer.

DETAILED DESCRIPTION

FIG. 1 illustrates a system 10 for producing gas from a reservoir 12 through a boundary layer 14 in accordance with one embodiment of the disclosure. In this embodiment, the reservoir 12 may be an unconventional reservoir. An unconventional reservoir is a coal, shale, or other tight gas and/or continuous gas formation. In a particular embodiment, the reservoir 12 is a coal layer, or seam. Coal seam is any formation of coal and may have a thickness of a few feet to a few hundred feet or otherwise. The coal seam may be a low permeability coal having a permeability value below 3 millidarcies (md) or an ultra-low permeability coal having a permeability of less than 1 millidarcy. The coal seam may in other embodiments have medium or other suitable permeability.

The reservoir 12 may comprise one or multiple layers of a same, similar or different formations. The reservoir 12 stores reservoir fluid, which may comprise gas and/or water. In the coal seam embodiment, the gas may comprise coal bed methane (CBM) gas. The coal seam may also comprise water and/or other fluids. Other reservoirs 12 may store other suitable types of gas and/or other fluids.

The boundary layer 14 may comprise one or a plurality of formations hydraulically interconnected and operable together communicate fluids from the reservoir 12 to one or more wells 20 in the boundary layer for production. The boundary layer 14 may be adjacent to the reservoir 12 or may be separated by one or more intermediate layers.

The boundary layer 14 is hydraulically connected to the reservoir 12. The boundary layer 14 and the reservoir 12 are hydraulically connected when fluid may travel from the reservoir 12 to the boundary layer 14 or otherwise suitably between the reservoir 12 and boundary layer 14 such that a significant portion of gas in the reservoir 12 may be produced in less time through the boundary layer 14 than directly through the reservoir 12.

In one embodiment, the boundary layer 14 has a permeability greater or significantly greater than the reservoir 12. For example, the boundary layer 14 may have a medium permeability of 3-7 md or a high permeability above 7 md. The boundary layer 14 may comprise a fractured formation including the plurality of fractures 16. The fractures 16 may be natural and comprise bedding planes, cleats, and/or stress fractures. In a particular embodiment, the boundary layer 14 may comprise a fractured, or cracked sandstone. The boundary layer 14 may in other embodiments comprise, for example, a shale or siltstone. The fractures 16 may extend horizontally, vertically and/or otherwise and may have, for example, a spacing of 3 meters to less than 1 centimeter. The fractures 16 may extend from the boundary layer 14 into the reservoir 12.

The boundary layer 14 may comprise water, gas and/or other fluids. For example, boundary layer 14 may have an initial gas content due to some natural, but limited, migration of gas from the reservoir 12 to the boundary layer 14 over an extended period of time. In this embodiment, the boundary layer 14 may have a gas content that is less than half and may be less than a quarter to one tenth than that of the reservoir 12. Accordingly, the reservoir 12 stores all, substantially all, at least seventy-five percent or a majority of the combined gas in the reservoir 12 and boundary layer 14. The boundary layer 14 may have no initial or no significant initial gas content. The boundary layer 14 may comprise a non-hydrocarbon layer in that it is not a source rock for gas or other hydrocarbons and comprises only traces of gas or no significant gas or producible gas for a viable project.

The wells 20 are drilled or otherwise provided from the surface 22 to the boundary layer 14. The wells 20 are drilled for the primary and/or initial purpose of producing gas and/or other fluids from the reservoir 12. The wells 20 may have other suitable purposes, for example, sequestration after gas production. The wells 20 may each be vertical, horizontal and/or otherwise suitably configured. As used herein, each means at least a subset of the identified items. For example, the wells 20 may slant or deviate from vertical. The wells 20 may include laterals and/or be fractured to connect to natural fractures within the boundary layer 14. For example, wells 20 may include short laterals of 200 feet or less, long laterals up to 500 feet and/or extended laterals having a length greater than 500 feet.

In one embodiment, one or more of the wells 20 may comprise cavity wells. In this embodiment, the wells 20 each include a cavity 24 having an enlarged volume within the wellbore. In one embodiment, the cavity 24 may be positioned within, substantially within, or otherwise at the level of, the boundary layer 14. The cavity 24 may have a diameter of, for example, eight feet. Other suitable cavity sizes and configurations may be used.

The wells 20 may each include a sump 26, or rat hole. The sump 26 may collect coal fines or other debris received by the well 20. The cavity 24, sump 26 and/or other part of the well 20 may extend to the reservoir 12. In another embodiment, the wells 20 may terminate within the boundary layer 14.

A pumping system 30 may be installed in each well 20. The pumping system 30 may comprise a rod pump, a progressive cavity pump, a down-hole motor pump, gas lift or other suitable system for removing water or other fluids from or otherwise moving fluids within the well 20. For example, the water may be removed to the surface for processing, transport, and/or re-injection. In another embodiment, the water may be injected into a disposal zone. Gas may be produced up the annulus of the well.

Also, pumping systems 30 may each have an inlet positioned in, or at the level of, boundary layer 14. In the cavity well embodiment, the inlets may be positioned within the cavities 24. In the down-hole motor embodiment, the down-hole motor may also be positioned within the cavity 24.

Wells 20 may be spaced based on the size, depth and/or gas content of the reservoir 12 and/or on the characteristics of the boundary layer 14, including fracture 16 spacing and direction. For example, a number of core, 2 inch or less, or other slim holes may be drilled with a core or other rig and/or air drilled. Slim holes intersecting a high permeability area of, for example, 1 Darcy to multiple Darcies may be redrilled as wells 20 or used to drill and/or locate wells 20. Thus, spacing may be irregular. Permeability may, in one embodiment, be high when air drilling is drowned out by water flowing into the slim hole. The slim hole testing, cavities 24, laterals and/or fracture stimulation projects may be sized and/or performed to enhance, maximize or optimize interception by the wells 20 of the fractures 30 in the boundary layer 14.

In operation, each pumping system 30 is disposed within a well 20 and used to create a pressure sink in the boundary layer 14 and/or reservoir 12. The pressure sink is an area around the well 20 having a pressure that is lower than that of other areas of the boundary layer 14 and/or reservoir 12. For example, each well may have a low pressure of less than 150 psi. Pressure in the boundary layer 14 may increase radially outwardly from the well 20. The pressure sink may be formed by reducing or otherwise drawing down the pressure and/or fluids 32 in the boundary layer 14. As previously described, the fluids 32 may be produced to the surface or, for byproducts, injected into a disposal zone.

The drawdown of the boundary layer 14 initiates or at least accelerates migration of reservoir fluids 34 from the reservoir 12 to the boundary layer 14. The migration is accelerated when it is at least twice and in some embodiments ten-fold or more the rate of any naturally occurring migration from the reservoir 12 to the boundary layer 14 prior to the drawdown. Gas and other fluids 34 migrate from the reservoir 12 to the boundary layer 14 through the large contact area between the formations. Contact area between the formations is the area of the formations in hydraulic connection and/or naturally occurring fractures that extend from one layer to another.

Once in the boundary layer 14, the reservoir fluids 34 may more easily and/or readily flow to the wells 20 based on the higher permeability of the boundary layer 14 than the reservoir 12. Thus, the large contact area between the boundary layer 14 and reservoir 12 may be used to communicate a large volume of fluids 34 from the reservoir 12 to the boundary layer 14 at an accelerated rate and the higher permeability boundary layer 14 thereafter used to communicate the reservoir fluids 34 to the wells 20 for production to the surface 22.

FIG. 2 illustrates a system 50 for producing a reservoir 52 through a plurality of boundary layers 54. In this embodiment, the reservoir 52 may be a coal seam as previously described in connection with FIG. 1. The boundary layers 54 are hydraulically connected to the reservoir 52 as previously described for reservoir 12 and boundary layer 14 in connection with FIG. 1. Boundary layers 54 may comprise a formation having a permeability higher than that of the reservoir 52.

Wells 60 may be vertical or other suitable bores that extend from the surface 62 through the upper boundary layer 54 and the reservoir 52 and into the lower boundary layer 54. The wells 60 may each comprise a cavity 64 and sump 66 formed in the lower boundary layer 54. The wells 60 may include laterals 68. The wells 60 may include laterals 68 when the laterals 68 are drilled from or connect to wells 60 or when wells 60 deviate to or significantly toward horizontal. Laterals 68 may have any suitable lengths. Wells 60, may each comprise a pumping system 68 as previously described in connection with wells 20 of FIG. 1.

In the embodiment of FIG. 2, boundary layer fluids 70 flow into wells 60 to create a pressure sink in the boundary layers 54. Reservoir fluids 72 flow into the wells 60 both directly and through boundary layers 54. For example, reservoir fluids in the vicinity of the wells 60 may flow directly into the well 60 while the remainder of the reservoir fluids 72 flow toward the pressure sinks of the boundary layers 54. Thus, the majority of reservoir fluids 72 flow to the wells 60 through the boundary layers 54 with, for example, 10, 20, 30 or other minority percentage flowing directly into wells 60. The gas is produced to the surface. Water and/or other liquids are collected in the cavity 64 and removed by pumping to the surface or disposal zone. Gas production may be thus accelerated and/or enhanced by flowing a substantial or majority amount of the gas through large contract areas between the reservoir 52 and the boundary layers 54 and through the relatively high permeable boundary layers 54 to wells 60.

FIG. 3 illustrates one embodiment of a pattern 80 of wells 20 for producing a reservoir 12 through a boundary layer 14. In this embodiment, wells 20 may be drilled in line with successive lines staggered. The wells 20 may be equally, substantially equally or otherwise suitably spaced. Other suitable well 20 patterns include a pattern based on high permeability areas of the boundary layer 14, for example, fractures 16.

FIG. 4 illustrates a method for producing gas from a reservoir 12 through a boundary layer 14 in accordance with one embodiment. The method begins at step 100 wherein a well spacing is determined. As previously described, well spacing may be determined based on fracture distribution and/or density of the boundary layer 14, other suitable characteristics of the boundary layer 14 and/or reservoir 12, and/or slim or other test hole results. In a particular embodiment, for a sandstone boundary layer having an average fracture spacing of three-meters, core holes of less than 2 inches may be drilled with air with a core rig and/or otherwise to determine location of the wells 20. Thus, determination of well spacing may include determining well locations based on permeability indicators for the boundary layer 14. The well spacing is set to, based on or otherwise designed for producing the reservoir 14 through the boundary layer 14.

Proceeding to step 102, wells 20 are drilled into the boundary layer 14. The wells 20 may be drilled for the primary purpose of producing reservoir fluid. Wells 20 may terminate in the boundary layer 14 or extend into the reservoir 12. In an embodiment where the boundary layer 14 is below the coal seam 12, the wells 20 may extend through the reservoir 12 and into the boundary layer 14. The wells 20 may be drilled using conventional and/or overbalanced techniques and/or underbalanced techniques. The cavity 24 may be formed using a flail arm, panagraph or other underreamer.

At step 104, the boundary layer 14 in the vicinity of the wells 20 is drawn-down to create a pressure sink and accelerate reservoir fluid 34 migration from the reservoir 12 to the boundary layer 14. Fluid migration may be accelerated when fluid migration from the reservoir 12 to the boundary layer 14 increases in rate and/or significantly in rate, for example, 2 to 10 times or more, in response to the pressure sink. The boundary layer 14 may be drawn-down by producing or otherwise removing fluid 34 from the boundary layer 14. For example, initial fluids in the boundary layer 14 may be produced to the surface 22 or otherwise suitably removed.

At step 106, fluid 34 migrates from the reservoir 12 to the boundary layer 14 due to, for example, a pressure differential between the reservoir 12 and the boundary layer 14. The pressure differential pressure sink may be otherwise suitably induced. The fluid 34, including gas and/or water migrates across the contact area between the reservoir 12 and boundary layer 14, which may include an intermediate layer, and through fractures 32 in the boundary layer 14 to wells 20.

At step 108, migrating water and gas are produced through wells 20 to the surface 22. Accordingly, all, a majority or other substantial amount of the gas of the reservoir 12 is intentionally or otherwise purposefully produced through the boundary layer 14 to, for example, accelerate and/or enhance gas production from the reservoir 12.

A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims. 

1. A method for producing a reservoir, comprising: drilling a well into a boundary layer of the reservoir for the purpose of producing reservoir fluid from the reservoir, the boundary layer hydraulically connected to the reservoir; accelerating migration of reservoir fluid from the reservoir to the boundary layer; and producing through the well reservoir fluid.
 2. The method of claim 1, wherein the well comprises a cavity well having a cavity formed in the boundary layer.
 3. The method of claim 1, wherein the well terminates in the boundary layer.
 4. The method of claim 1, wherein migrating reservoir fluid from the reservoir to the boundary layer comprises migrating water and gas from the reservoir to the boundary layer, further comprising pumping water to the surface through a pump having an inlet in the boundary layer.
 5. The method of claim 1, wherein the reservoir comprises an unconventional reservoir.
 6. The method of claim 5, wherein the reservoir comprises a coal formation.
 7. The method of claim 6, wherein the reservoir fluid comprises gas.
 8. The method of claim 1, wherein the boundary layer comprises a fractured layer.
 9. The method of claim 1, wherein the boundary layer comprises a sandstone.
 10. A system for producing a reservoir, comprising: a subsurface reservoir; a boundary layer hydraulically connected to the reservoir; a plurality of wells extending from a surface into the boundary layer, the boundary layer comprising a non-hydrocarbon layer; and a pumping system disposed in each well; a pressure sink generated at each well; and wherein reservoir fluid migrates from the reservoir to the boundary layer in response to at least the pressure sink.
 11. The system of claim 10, wherein the reservoir comprises an unconventional reservoir.
 12. The system of claim 11, wherein the unconventional reservoir comprises a coal seam.
 13. The system of claim 10, wherein the boundary layer comprises a fractured layer.
 14. The system of claim 10, wherein the boundary layer comprises a sandstone.
 15. The system of claim 10, wherein the pumping system comprises a progressive cavity pump.
 16. The system of claim 10, wherein the pumping system comprises a downhole motor.
 17. The system of claim 10, wherein the pumping system comprises a rod pump.
 18. The system of claim 10, wherein the wells comprise vertical wells.
 19. The system of claim 10, wherein the wells comprise cavity wells.
 20. The system of claim 10, wherein the wells terminate in the boundary layer.
 21. A method for producing coal bed methane (CBM) through a boundary layer, comprising: providing a plurality of wells extending from a surface into a boundary layer of a coal formation; the boundary layer hydraulically connected to the coal layer; removing fluids collected in the wells to cause accelerated migration of CBM from the coal layer to the boundary layer; and producing the CBM through the wells.
 22. The method of claim 21, wherein the boundary layer comprises a fractured formation.
 23. The system of claim 22, wherein the boundary layer comprises a fractured sandstone.
 24. The method of claim 21, further comprising removing the water collected in the well by pumping the water to the surface.
 25. The method of claim 21, wherein the boundary layer is adjacent to the coal layer.
 26. The method of claim 21, wherein the plurality of wells are arranged in a pattern.
 27. The method of claim 21, wherein the wells include at least one lateral in the boundary layer. 